1. Field of the Invention
The present invention relates to a frame structure suitable for use in a motorcycle or similar vehicle.
2. Description of Background Art
One conventional frame structure which is suitable for a motorcycle is disclosed in Japanese patent publication No. 4-71747 and includes a main frame in the form of a round pipe extending rearwardly from a head pipe on which a front wheel is steerably supported. A pivot frame is provided which includes an upper end welded to a side of a rear end of the main frame and extends downwardly. A seat rail includes a front end welded to a side of the rear end of the main frame and extends rearwardly. The upper end of the pivot frame has an axis extending vertically and crossing the axis of the main frame and the axis of the front end of the seat rail.
The pivot frame and the seat rails are formed of round pipes. The pivot frame has an intermediate pivot to which the front end of a rear arm is attached by a shaft, with a rear wheel supported on the rear end of the rear arm. A seat is supported on the seat rail.
Because the axis of the pivot frame at its attachment region crosses the axis of the main frame, the pivot frame and the seat rail are welded in different directions, respectively. Since the pivot frame and the seat rail need to be installed by the use of different welding processes, the entire welding operation is complex and carried out in a large number of steps, and cannot easily be automated.
Inasmuch as the pivot frame and the seat rail are welded separately from each other, different accuracy levels of the respective welding processes tend to greatly affect the overall welding accuracy.
The main frame in the form of a round pipe makes it difficult to fabricate an abutment structure of the upper end of the pivot frame due to the accuracy requirement for a cut edge thereof. Furthermore, it is difficult to weld the pivot frame to the main frame because the pivot frame needs to be welded along a three-dimensional line in the circumferential direction of the main frame. The welding line cannot sufficiently be increased even though increasing the welding line is desirable for increasing the welding strength.